1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive for circular self servo writing and spiral self servo writing using prewritten reference servo bursts.
2. Description of the Prior Art
Disk drives are servo written with embedded servo sectors to enable accurate positioning of the head with respect to the data tracks. The embedded servo sectors comprise coarse positioning information (e.g., a data track number) together with fine positioning information for tracking the centerline of the data track while writing data to the disk and reading data from the disk. The fine positioning information comprises servo bursts located at precise intervals and offsets with respect to a data track's centerline.
FIG. 1 which shows a disk 2 comprising a number of concentric data tracks 4 having a number of embedded servo sectors 6 which form a servo wedge from the outer diameter track to the inner diameter track. Each servo sector 6 comprises a preamble 8, a sync mark 10, servo data 12, and servo bursts 14. The preamble 8 comprises a periodic pattern which allows proper gain adjustment and timing synchronization of the read signal, and the sync mark 10 comprises a special pattern for symbol synchronizing to the servo data 12. The servo data 12 comprises identification information, such as sector identification data and a track address. The servo control system reads the track address during seeks to derive a coarse position for the head with respect to the target data track. The track addresses are recorded using a phase coherent Gray code so that the track addresses can be accurately detected when the head is flying between data tracks. The servo bursts 14 in the servo sectors 6 comprise groups of consecutive transitions (e.g., A, B, C and D bursts) which are recorded at precise intervals and offsets with respect to the data track's centerline. Fine head position control information is derived from the servo bursts 14 for use in centerline tracking while writing data to and reading data from the target data track.
Conventional disk drives are servo written using servo writer machines which processes the disk drives in assembly line fashion during manufacturing. The servo writers employ very precise head positioning mechanics, such as a laser interferometer, for positioning the head at precise radial locations with respect to previously servo-written tracks so as to achieve very high track densities. In addition, the head disk assembly (HDA) within the disk drive is typically exposed to the environment through apertures which allow access to the disk drive's actuator arm and the insertion of a clock head. This requires the servo writing process to take place in a “clean room” free of contaminant particles. Further, the manufacturing throughput is limited by the number of servo writers available, and the cost of each servo writer and clean room becomes very expensive to duplicate.
It is known to “self servo write” a disk drive using the internal components of the drive so as to obviate the need for external servo writers, thereby decreasing the manufacturing cost and increasing manufacturing throughput. Many of the self servo writing techniques suggest to propagate a write clock as well as the servo sectors from a previously servo-written track to a next servo-written track. However, these prior art techniques are susceptible to exponential error growth due to the multiplicative effects of propagating the write clock and servo sectors thousands of times.
There is, therefore, a need for an improved technique for servo writing a disk drive which reduces the bottleneck of external servo writers and ameliorates the exponential error growth inherent in the prior art self servo writing techniques.